Nikon D3X User Manual page 110

The Electronic Rangefinder
If the lens has a maximum aperture of f/5.6
or faster, the viewfinder focus indicator can
be used to confirm whether the subject in
the selected focus point is in focus (the
focus point can be selected from any of the
51 focus points). After positioning the
subject in the selected focus point, press
the shutter-release button halfway and
N
rotate the lens focusing ring until the in-
focus indicator (I) is displayed (pg. 55).
Note that with the subjects listed on page
82, the in-focus indicator may sometimes
be displayed when the subject is not in
focus; confirm focus in the viewfinder
before shooting.
A
Focal Plane Position
To determine the distance between your subject
and the camera, measure from the focal plane
mark (E) on the camera body. The distance
between the lens mounting flange and the focal
plane is 46.5 mm (1.83 in.).
Focal plane mark
84